It is imperative that you push through.
Right now, we're only on day five, and I feel like I could walk away from this project and not look back. I knew I was going to feel that way, so I'm okay with it. There are other responsibilities I could walk away from too if I didn't feel obligated to do them, such as organizing my desktop, washing the salt off my car in the winter, or having a wisdom tooth removed. This is the week during which writers who are doing this for the first time are going to find out that writing isn't all fun and games. (If you're a writer year round, you probably know this already.) Pretty much, writing is blood, sweat, and tears. You put your lifeblood into the book, you sweat profusely when you feel stuck, and you cry buckets when not even you like the end results.
So for those of you beginning to feel like this isn't the fun you thought it would be, hang in there. There are going to be days that writing your Nanobook doesn't feel like anything even resembling fun, but when you hold that manuscript in your hands at the end of thirty days and know that you did it along with hundreds of fellow Nanowriters, you'll be more than glad that you didn't give up.